The P-Answer-State Header Extension to the Session Initiation Protocol for the Open Mobile Alliance Push to Talk over Cellular
RFC 4964

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Network Working Group                                      A. Allen, Ed.
Request for Comments: 4964                      Research in Motion (RIM)
Category: Informational                                          J. Holm
                                                                Ericsson
                                                               T. Hallin
                                                                Motorola
                                                          September 2007

 The P-Answer-State Header Extension to the Session Initiation Protocol
        for the Open Mobile Alliance Push to Talk over Cellular

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document describes a private Session Initiation Protocol (SIP)
   header (P-header) used by the Open Mobile Alliance (OMA) for Push to
   talk over Cellular (PoC) along with its applicability, which is
   limited to the OMA PoC application.  The P-Answer-State header is
   used for indicating the answering mode of the handset, which is
   particular to the PoC application.

Allen, et al.                Informational                      [Page 1]
RFC 4964               The P-Answer-State Header          September 2007

Table of Contents

   1. Introduction ....................................................3
   2. Overall Applicability ...........................................3
   3. Terminology .....................................................3
   4. Background for the Extension ....................................4
   5. Overview ........................................................5
   6. The P-Answer-State Header .......................................6
      6.1. Requirements ...............................................8
      6.2. Alternatives Considered ....................................8
      6.3. Applicability Statement for the P-Answer-State Header ......9
      6.4. Usage of the P-Answer-State Header ........................10
           6.4.1. Procedures at the UA (Terminal) ....................11
           6.4.2. Procedures at the UA (PTT Server) ..................11
           6.4.3. Procedures at the Proxy Server .....................14
   7. Formal Syntax ..................................................14
      7.1. P-Answer-State Header Syntax ..............................14
      7.2. Table of the New Header ...................................14
   8. Example Usage Session Flows ....................................15
      8.1. Pre-Arranged Group Call Using On-Demand Session ...........15
      8.2. 1-1 Call Using Pre-Established Session ....................21
   9. Security Considerations ........................................28
   10. IANA Considerations ...........................................28
      10.1. Registration of Header Fields ............................28
   11. Acknowledgements ..............................................29
   12. References ....................................................29
      12.1. Normative References .....................................29
      12.2. Informative References ...................................30

Allen, et al.                Informational                      [Page 2]
RFC 4964               The P-Answer-State Header          September 2007

1.  Introduction

   The Open Mobile Alliance (OMA) (http://www.openmobilealliance.org) is
   specifying the Push to talk Over Cellular (PoC) service where SIP is
   the protocol used to establish half-duplex media sessions across
   different participants.  This document describes a private extension
   to address specific requirements of the PoC service and may not be
   applicable to the general Internet.

   The PoC service allows a SIP User Agent (UA) (PoC terminal) to
   establish a session to one or more SIP UAs simultaneously, usually
   initiated by the initiating user pushing a button.

   OMA has defined a collection of very stringent requirements in
   support of the PoC service.  In order to provide the user with a
   satisfactory experience, the initial session establishment (from the
   time the user presses the button to the time they get an indication
   to speak) must be minimized.

2.  Overall Applicability

   The SIP extension specified in this document makes certain
   assumptions regarding network topology and the existence of
   transitive trust.  These assumptions are generally NOT APPLICABLE in
   the Internet as a whole.  The mechanism specified here was designed
   to satisfy the requirements specified by the Open Mobile Alliance for
   Push to talk over Cellular for which either no general-purpose
   solution was found, where insufficient operational experience was
   available to understand if a general solution is needed, or where a
   more general solution is not yet mature.  For more details about the